From Longman Dictionary of Contemporary Englishbunbun /bʌn/ noun [countable] 1 British EnglishDF a small round sweet cake a sticky bun2 DFa small round type of bread a hamburger bun3 
DCBif a woman has her hair in a bun, she fastens it in a small round shape at the back of her head4 → buns5 → have a bun in the oven
DCBif a woman has her hair in a bun, she fastens it in a small round shape at the back of her head4 → buns5 → have a bun in the ovenExamples from the Corpus
bun• This species looks rather like a bun, but not highly convex, and with a five-sided outline.• Spoon into the choux buns and sprinkle with paprika.• She so obviously hadn't given up stuffing herself with Salmon's cream buns.• Gao Yang counted seven old fingers buried in the steamed bun, which had long since given up its original shape.• The blanched nape of a neck, spiders of hair breaking free of the bun, twirling on the surface.• While the buns were cooking members were divided into teams.Origin bun 1. (1300-1400) Perhaps from Old French bugne “swelling (caused by a blow), type of fritter”2. (1800-1900) Perhaps from Scottish Gaelic bun; → BUNNY 